There are numerous prior art methods for coating substrates to improve their performance, e.g. lifetime, abrasion wear resistance and similar properties. For example, consider the case of plastic sunglass lenses or plastic prescription eyewear. Due to the ease of scratching plastic, abrasion-resistant coatings are deposited onto the surface of plastic lenses. These hard outer coatings increase the useful life of the lenses. To make such coatings marketable, the process for depositing these hard coatings must be inexpensive, reliable and reproducible.
Plastic lenses sold into the ophthalmic lens market are largely coated by acrylic and polysiloxane dip-coatings or spin coatings. These coatings significantly improve the abrasion resistance of the lens compared to the uncoated lens. This is particularly true for the case of polycarbonate which is very subject to abrasion. However, improved abrasion resistance of coated lenses is still a major problem in the ophthalmic lens industry. The industrial goal is to obtain plastic lenses which exhibit the same abrasion resistance as glass lenses. Current commercial plastic lenses have abrasion resistance characteristics which are poor compared to optical glass. Therefore, when purchasing lenses, one must choose between glass, which is very abrasion resistant but is heavier, or plastic which is lighter but much less abrasion-resistant.
While glass is roughly 30 times harder than plastic, there are many applications for glass where the lifetime of the product is restricted by the limited abrasion resistance of this material. Such products include architectural glass, analytical instrument windows, automotive windshields, and laser bar code scanners for use in retail stores and supermarkets. For example, the read failure rates of uncoated bar code scanner windows in typical grocery store checkout lanes increase noticeably within a matter of weeks due to damage by impacting cans and bottles.
Various coatings made by techniques other than the wet chemical methods have been suggested for plastic substrates, including lenses for optical products. Most of these coatings are so-called "plasma polymers" which are largely produced by creating a plasma from siloxane precursor gases. The substrates are exposed to the plasma, but they are not biased to cause energetic ion bombardment. The performance of these plasma polymers is often only marginally better than that of the polysiloxane and acrylic spin and dip coatings, and the performance of these coatings does not approach the performance of glass. These films are often quite soft and are not useable as protective coatings except on extremely soft substrates.
Other coating processes have been suggested in which energetic ion bombardment is caused by mounting the substrates on the powered electrode in a radio frequency (RF) plasma system and exposing the parts to the plasma, thereby creating a negative bias on the substrate surface. The resultant coatings are often more abrasion resistant than the "plasma polymers". These plasma systems are not readily scaled to a throughput required for mass production nor are they easily operated in a reproducible, controlled fashion in a production environment. The RF plasma process also suffers in that the deposition process, and the properties of the resultant coating are dependent on whether the substrate to be coated is an electrical conductor or insulator. Furthermore, if the substrate is an insulator, the thickness of the substrate strongly influences the deposition process energetics and the properties of the resultant coating. This means that for production coating of insulating substrates of different size and shape, e.g. plastic lenses, it may be necessary to have different coating processes for each type of substrate. This reduces the flexibility of the process for use in production. Additionally, systems with large area electrodes are not widely available. For example, there are no readily available commercial parallel plate RF deposition systems having large electrodes, i.e. at least one meter in diameter.
The following references illustrate prior art coating processes in which plasmas are used in direct contact with the surface of the substrate:
Rzad et. al., U.S. Pat. No. 5,156,882, describe a method of preparing a transparent plastic article having an improved protective stratum thereon. The protective stratum is deposited by plasma enhanced chemical vapor deposition (PECVD).
Balian et. al., U.S. Pat. No. 5,206,060, describe a process and device for depositing thin layers on a substrate using a plasma chemical vapor deposition (PCVD) technique. The substrate must be made conductive, and is used as an electrode in the PCVD process.
Reed et. al., U.S. Pat. No. 5,051,308, describe an abrasion-resistant article and a method for producing the same. The article includes a plastic substrate and a gradational coating applied by a PECVD process.
Devins et. al., U.S. Pat. No. 4,842,941, also describe an abrasion-resistant article and a method for making the same. The article includes a polycarbonate substrate, an interfacial layer of an adherent resinous composition on the substrate, and an abrasion-resistant layer applied on top of the interfacial layer by PECVD.
Brochot et. al., U.S. Pat. No. 5,093,153 describe a coated object comprising a glass substrate coated with an organomineral film by a PECVD process.
Kubacki, U.S. Pat. No. 4,096,315, describes a low-temperature plasma polymerization process for coating an optical plastic substrate with a single layer coating for the purpose of improving the durability of the plastic.
Enke et. al., U.S. Pat. No. 4,762,730, describe a PECVD process for producing a transparent protective coating on a plastic optical substrate surface.
All of the prior art plasma deposition methods for application of wear and abrasion-resistant coatings suffer from one or more of the following deficiencies and shortcomings:
(1) difficulty in pre-cleaning of substrates prior to deposition; PA1 (2) adhesion of the protective, abrasion-resistant coating; PA1 (3) permeation of the coatings by water vapor and oxygen; PA1 (4) fabrication of coherent, dense coatings; PA1 (5) control of coating properties during a deposition run and batch-to-batch variation of coating characteristics; PA1 (6) coating thickness control and reproducibility of thickness; PA1 (7) part-to-part and batch-to-batch control of coating uniformity; PA1 (8) difficulty in coating substrates of complex geometry or configuration; and PA1 (9) production readiness and ability to scale-up the deposition process for mass production.
These shortcomings are highlighted in the following review of the two preferred prior art methods for deposition of abrasion-resistant coatings on plastic optical substrates: plasma polymerization and biased RF plasma deposition.
The first problem encountered by both methods is the difficulty in pre-cleaning the substrates prior to deposition of the adhesion layer or abrasion-resistant film. Typically substrates are pre-cleaned in an inert gas or glow discharge (plasma) prior to deposition. This pre-cleaning technique suffers from low cleaning rate, and re-contamination of the substrate by sputtered contaminants which are deposited back onto the substrate.
One of the key requirements for a protective coating on a variety of substrates, including optics, is the need to provide a barrier to moisture, oxygen, and other environmental elements. This requires formation of a coating structure with optimal atom packing density. This atom packing density is maximized by a high degree of ion bombardment during film growth, which is not easily attainable or optimized by the plasma polymerization methods of the prior art.
Regarding the control of the coating properties within a single deposition run, and from batch-to-batch, it is well known that control is difficult with the plasma deposition methods. For the case of deposition of electrically insulating coatings on electrically conductive substrates by the biased RF plasma technique, it is known that as the deposited coating thickness increases, there will be a gradual decrease of the surface bias on the growing film; see Meyerson et al., U.S. Pat. No. 4,647,494, column 6, line 67 through column 7, line 3. This decrease results in a change in the properties of the deposited coating, i.e. hardness, stress and hydrogen concentration.
Because the size and shape of the particular part to be coated, and its method of fixturing influence the plasma uniformity and plasma density around the part, it is difficult to predict and control deposition thickness uniformity across multiple parts coated within a single coating run using the plasma deposition methods of the prior art.
While the plasma deposition methods offer high deposition rates, it is difficult to reproducibly control deposition rate, deposition thickness and deposition uniformity across large areas with plasma deposition methods. Because of the interdependence of process variables such as pressure, gas flow rate, power, and substrate bias, accurate control of deposition thickness is difficult. Thus, it is very difficult to manufacture coating layers with thickness less than 0.1 micron, and with run-to-run thickness variation of less than approximately 10%. This is a significant disadvantage of the plasma deposition techniques of the prior art for the deposition of optical coatings, especially those requiring the use of multiple, thin layers of varying refractive index, such as antireflection coatings.
Finally, because of the sensitivity of the plasma deposition processes to substrate geometry, it is often impossible to coat parts of complex geometry or configuration. Examples of complex geometry include optical lenses with high corrective power which may be edged to a variety of shapes, industrial molds used to fabricate plastic parts, and other industrial machine parts, including shafts, gears, bearings, and the like. The current industrial trend is to fabricate many of these industrial machine parts from electrically insulating plastics and ceramics. These electrically insulating industrial machine parts are especially difficult to coat uniformly by the plasma deposition methods.
All of the difficulties above combine to make mass production of protective, abrasion-resistant coatings on a variety of substrates by the plasma deposition processes of the prior art very problematic indeed. Clearly, an improved method for flexible, reproducible, and high quality mass production of abrasion-resistant coatings has long been sought.
Ion beam etching and deposition of many materials is known in the prior art. For example, ion milling is commonly used in semiconductor processing. Ion beam systems typically are more controllable than RF plasma systems in that the deposition and etching process parameters, e.g. plasma potential, substrate bias, plasma current, gas flows and chamber pressures are not as strongly coupled as they are in the RF plasma process. This results in a wider process window and better control for ion beam processing, as compared to plasma processing. Additionally, ion beam deposition equipment is available which is capable of processing in excess of 1000 square inches of substrate material per batch. It is believed that RF equipment is not commercially available which approaches this level of scale. The combination of the higher degree of control for ion beam processing and the ability to scale to large areas allows for a process which is more easily moved into production and is more robust. However, one major disadvantage to prior art ion beam deposition processes, e.g. for deposition of DLC films, is their relatively low deposition rate which leads to long production times for thick coatings, and hence high production cost.
In an article published in Clinical Materials, Vol. 12, pages 237-244 (1993), G. Dearnaley describes a process in which low vapor pressure materials are condensed on the surface of the article to be coated and simultaneously bombarded by a high energy nitrogen ion beam. In this case, the ion energy required is greater than 10 kV. These large voltages are difficult to control and become problematic in a production environment. In addition, the coatings manufactured by this method are opaque and not useable for applications where a transparent coated product is required.
Kimock, et al., U.S. Pat. Nos. 5,135,808, 5,190,807, 5,268,217 disclose direct ion beam deposition processes using a hydrocarbon gas or carbon vapor for producing abrasion wear resistant products comprising substrates with hard outer coatings of substantially optically transparent diamond-like carbon (DLC) useful for commercial articles such as optical lenses, sunglass lenses, and bar code scanner windows.